This invention relates to apparatus which is capable of recovering the signals from N sites, N.gtoreq.2, distributed along a single coaxial cable. This multiplexing technique allows the separation of signals from many transducers on one line.
In the prior art, there are three methods of multiplexing signals: time division, frequency division, and spatial division. These techniques all require considerable hardware at each point along the cable where a signal is originated. Time division multiplexing requires that accurate timing be present at each signal source, and necessitates the use of high speed analog switches to gate the signal on the cable at the appropriate times. For frequency division multiplexing, each signal source must have associated hardware for frequency translation. Spatial division multiplexing used in the context of cables corresponds to multiconductor cable. Each signal has a separate path to the receiver. For applications where data originates from many points along the cable, all these techiques require either large multiconductor cables, considerable electronics at each source, or both.
This invention offers the alternative to the techniques discussed hereinabove. It involves the use of time domain reflectometry (TDR) and variable resistive elements distributed along the cable. Each resistive element is variable and each one is distributed somewhere along the cable. Any of the N resistive elements may change its resistance, depending on the instantaneous value of the signal at its input, which generally is a continuous signal. This continuous signal is pulsed at intervals by an interrogation signal generated by the time-domain reflectometer, the pulses being sent down the line, a very small portion of the pulse being reflected back to its source.